This invention relates to thermosetting coating compositions. More particularly, the invention relates to polymeric, high solids, thermosetting coating compositions adapted to provide an automotive top coat demonstrating hardness, high gloss, outstanding durability and excellent resistance to solvents and water. Still more particularly, the invention relates to a high solids, thermosetting coating composition which is adapted to be used as an automotive top coat and which is particularly adapted to be used as an automotive top coat wherein the top coat includes metallic flake as a pigment.
Because of increasingly strict solvent emissions regulations and the scarcity and expense of organic solvents in recent years, low solvent emission paints have become very desirable. A number of high solids paint compositions have been proposed to meet these low solvent emission requirements. However, many of these compositions are deficient because of difficulty in application, slow curing rates, lack of flexibility, poor durability and low solvent and water resistance. Many of the proposed compositions have been particularly deficient as automotive top coats, particularly when the top coat is to include metallic flake as a pigment.
To obtain reasonable application properties, prior art high solids coatings have generally been prepared from low viscosity resins. These low viscosity resins are either low molecular weight polymers or monomeric materials. All contain high functionalities. Upon baking or drying, they form a crosslinked network. Due to the low viscosity, however, these high solids coatings generally have a tendency to sag during application or baking. If the high solids coatings contain metallic pigments, such as aluminum flakes, aluminum reorientations such as mottling and reverse flop become major problems. This aluminum reorientation is not a problem in low solids coatings since high viscosity resins can be used. These high viscosity resins retard aluminum flake reorientation.
Because of the apparent necessity of using low viscosity resins, prior art compositions have not employed resins with high molecular weights or crosslinked structures as the principle binder polymer in high solids coatings.
Recently non-aqueous dispersion resin technology has been widely used in the automotive industry. These compositions are appealing due to their high molecular weight and fast solvent release. These non-aqueous dispersion resins or NAD's, as they are commonly called, however, are not used as the predominant film forming polymer and resultant compositions have relatively low solids content. NAD resins are generally prepared in inert solvents and they tend to be dissolved or swollen in active solvents, thus resulting in increased viscosity. Crosslinked dispersion resins have been found to have a more stable viscosity in active solvents.
Patents exemplifying prior art high solids coating compositions are: U.S. Pat. Nos. 3,960,979 and 4,018,848.
Patents exemplifying NAD-based coating compositions are: U.S. Pat. Nos. 3,317,635; 3,514,500; 3,607,821; 3,814,721; 3,966,667; and 4,147,688.